Amazing Sonic Boom Cloud!

This is a condensation cone. It comes not from the aircraft breaking the sound barrier, but forms when the aircraft is near the speed of sound. The air around the aircraft is going supersonic relative to the aircraft (from being pushed aside by the plane), but the aircraft itself doesn't need to be travelling at the speed of sound. You can learn more about this phenomenon at: https://en.wikipedia.org/wiki/Vapor_cone